A modular dc-dc conversion system to boost a voltage is disclosed in the following patent application:                Robert Erickson, “Integrated Photovoltaic Module” U.S. patent application Ser. No. 13/318,589, May 10, 2010, which is incorporated by reference herein in its entirety as if fully set forth herein.The approach in this reference provides a non-inverting buck-boost converter arranged in series with a unidirectional DC transformer (DCX) module. The reference does not disclose a DCX module whose output port is connected in series with the output of a converter module, such as a boost module.        
A publication that provides a detailed analysis of a DC transformer circuit, such as the DCX circuit shown in FIG. 8, including design information, is the following:                D. Jones and R. Erickson, “Analysis of Switching Circuits through Incorporation of a Generalized Diode Reverse Recovery Model into State Plane Analysis,” IEEE Transactions on Circuits and Systems I, vol. 60, no. 2, pp. 479-490, February 2013, which is incorporated by reference herein in its entirety as if fully set forth herein.        
A publication that describes a method for controlling buck and boost converters using pass-through modes is the following:                D. Jones and R. Erickson, “A Nonlinear State Machine for Dead Zone Avoidance and Mitigation in a Synchronous Noninverting Buck-Boost Converter,” IEEE Transactions on Power Electronics, vol. 28, no. 1, pp. 467-480, January 2013, which is incorporated by reference herein in its entirety as if fully set forth herein.        
A DC-DC boost converter increases a DC input voltage Vin to produce a DC output voltage Vout=MVin, where the conversion ratio M is greater than or equal to one. An example of a well-known implementation of a boost converter 10 is illustrated in FIG. 1. In this circuit 10, a controller circuit drives a transistor gate 12 with a repetitive signal that causes the transistor Q to be ON for a time DTs, and OFF for a time (1−D)Ts, where D is the transistor duty cycle and Ts is the switching period. When the transistor Q is ON, energy from an input source is stored in the inductor L. When the transistor Q is OFF, the diode D becomes forward-biased by an inductor current, and energy stored in the inductor L is released to the output. To the extent that the circuit elements have low power loss, the output voltage is given by Vout=Vin/(1−D), and the efficiency η=Pout/Pin can approach 100%. A bi-directional converter 20 that is an extension of the conventional DC-DC boost converter is illustrated in FIG. 2, in which a pair of transistors Q1 and Q2 and a pair of diodes D1 and D2 allow the inductor current to be either positive or negative, so that power can flow from either Vin to Vout or Vout to Vin.
It is well known that a variety of loss mechanisms reduce the efficiency of the boost converters of FIG. 1 and FIG. 2. These loss mechanisms can be broadly grouped into DC losses and AC losses. In this disclosure, DC losses refer to losses that do not depend directly on the switching frequency, such as losses arising from the forward voltage drops of the semiconductor devices and losses caused by the DC resistance of the inductor winding. AC losses refer to losses that increase with switching frequency, such as semiconductor switching losses caused by transistor and diode switching times, diode reverse recovery, semiconductor output capacitances, and transistor drive power. The inductor also exhibits AC losses caused by core loss as well as AC winding losses arising from the skin and proximity effects. As a result of these loss mechanisms, the conventional boost converter circuit may exhibit substantially degraded efficiency. Furthermore, the efficiency is a function of input and output voltage, switching frequency, and output power. FIG. 3 illustrates typical efficiency curves of a boost converter, for several values of resistive load. It can be seen that the efficiency degrades as the duty cycle (and hence also output voltage) is increased.
Typically, power converters are thermally limited by their cooling systems, and these cooling systems may have significant size and cost. For a given cooling technology and cooling system size, there is a fixed amount of loss that can be tolerated while maintaining an acceptable temperature rise. In a thermally limited system, improvement of efficiency means that the output power can be increased. For example, if the efficiency can be increased from 96% to 98%, then the loss is approximately halved. Assuming that the system is still thermally-limited and the cooling system size is maintained constant, then the rated output power can be doubled and the cost per watt of output power is halved. Ultimately, it is desirable to increase the ratio Pout/Ploss so that the converter cost per watt, or cooling system size and cost, are decreased.
A conventional boost converter also exhibits reduced efficiency at low output power, as a result of AC losses. Converter efficiency over a range of output powers and voltages is increasingly important because the converter may operate at partial power for a substantial fraction of the time. For example, power converters for solar power systems are characterized by a weighted efficiency that accounts for efficiency not only at rated power, but also at lower powers corresponding to less than full irradiance. Power converters for electric vehicle applications must operate over driving profiles having a wide variety of speeds and accelerations, corresponding to a variety of converter output voltages and powers; improvement of efficiency at all of these operating points is needed to improve the effective miles per gallon (MPGe) of the vehicle. Power converters for grid interface of wind turbines must also operate efficiently with a wide range of voltages and output powers, corresponding to a range of wind speeds. FIG. 4 illustrates a typical efficiency curve for a conventional boost converter, operating at a constant output voltage and with variable output power. It is desirable to increase the efficiency not just at maximum power, but also at lower powers.
AC switching losses can be reduced by reduction of the switching frequency. However, this necessitates use of larger inductor and capacitor elements, which are more expensive. The larger inductor may also exhibit higher DC resistance. Therefore, it is often undesirable to reduce the switching frequency, and solutions are needed that achieve high efficiency without sacrificing switching frequency.
The size of the output capacitor is often limited by its root-mean-square (RMS) current rating. The RMS capacitor current increases as the duty cycle is increased. To reduce the size and cost of this capacitor, an improved circuit is needed that can boost the voltage substantially, while maintaining relatively low RMS capacitor current.
High-voltage power semiconductor devices typically exhibit increased switching times and increased switching losses. In a boost converter system, for example, it may be desirable to avoid use of high-voltage semiconductor devices, employing multiple lower-voltage devices instead. A well-known example of this is a multilevel converter; a three-level boost converter is illustrated in FIG. 5. This converter circuit 30 can achieve some of the goals delineated here, including reduction of AC losses and use of semiconductors with reduced voltage rating. However, it operates with substantially increased capacitor RMS currents, and hence requires expensive capacitors.